A new approach to synthesize graphene is oxidizing graphite powder with a mixture of H2SO4/H3PO4 acids and potassium permanganate. Parameters such as reaction time, reaction temperature, and amount of concentration were varied to study the degree of oxidation of graphite to graphene oxide. Currently, an improved method for the preparation of graphene oxide was the most common one. A mixture of H2SO4/H3PO4 (9 : 1 volume ratio) instead of only H2SO4 resulted in increased hydrophilic and oxidized GO without the emission of toxic gas, which differs from the traditional Hummers’ method. The graphene oxide (GO) was converted to reduced graphene oxide (rGO) by chemical reduction using ascorbic acid as the reducing agent. The GO and rGO were characterized by UV-visible spectroscopy, FTIR spectroscopy, and X-ray diffraction patterns. The result showed that treating graphite powder with potassium permanganate (1 : 9) and a mixture of concentrated H2SO4/H3PO4 acids at 50°C for 12 hours resulted in a better oxidation degree. The designed synthesis strategy could be easily controlled and is an alternative green approach for the production of graphene oxide and reduced graphene oxide.
Graphene-based materials are playing a great role in the world in every aspect. Among these, graphene is one of the recent miracle materials. Especially, it has many applications in the electronics world since it is the most thinnest, transparent, strongest, and conductive material [
Synthesis of graphene oxide (GO) is achieved by placing graphite in concentrated acid in the presence of an oxidizing agent. The Tour method demonstrated a less hazardous and more efficient method for graphite oxidation. This and its modified versions are presently the most commonly used methods for the oxidation of graphite [
In the most successful cases, the chemical reduction of GO was conducted using hydrazine or hydrazine hydrate. However, these are highly poisonous and explosive [
Fernandez-Merino et al. revealed that GO reduced by vitamin C (VC) could achieve a C/O ratio of about 12.5 and a conductivity of 77 S/cm, which are comparable to those produced by hydrazine in a parallel experiment. In addition, VC has a great advantage of its nontoxicity in contrast to hydrazine and a higher chemical stability with water than NaBH4. Furthermore, the reduction in the colloid state does not result in the aggregation of rGO sheets as produced by hydrazine, which is beneficial for further applications [
The aim of this study is synthesis of reduced graphene oxide by the Tour method using the cheapest material, that is, graphite, as a starting material using different parameters such as the effect of concentration, temperature, and reaction time in the oxidation level of graphene oxide in a cost-effective way. This was mainly carried out using sulfuric acid, phosphoric acid, and potassium permanganate excluding sodium nitrate that emits toxic gases into the environment. This makes the method environmental friendly. Another most cost-effective and environmental friendly method is the reduction of graphene oxide into rGO (reduced graphene oxide) using ascorbic acid. Even though this method is green, it was not performed with different parameters. In this project, graphene oxide has been produced with different parameters.
Graphene has the potential of replacing the costly and brittle silicon-based electronic devices, but it is in its infant stage. This study is therefore significant for the following: contributes to the study of graphene oxide and reduced graphene oxide, shows how to produce graphene oxide from cheap graphite, shows the effect of parameters on the synthesis of graphene-based material, creates awareness about different applications of graphene-based materials, and provides information for future related studies.
Graphene oxide (GO) was prepared by mixing 90 mL of concentrated H2SO4 (sulfuric acid) and 10 mL of concentrated H3PO4 (
When 30 wt.% H2O2 was added, bubbling occurred and a bright yellow color was observed, indicating a high level of oxidation. The solution was then filtered using the filter paper to remove the metal sulfate and a graphite oxide (GTO) filter cake was produced. The cake was washed with 5% HCl aqueous solution until the sulfate ions are removed completely. The removal of the metal sulfate ions was confirmed using BaCl2 solution. The washing process was carried out repeatedly by centrifugation at 4000 rpm for 4 hrs, and the supernatant was decanted away. The pH of the collected material was checked using a universal indicator. The collected material (GTO) was stirred in distilled water at 60°C for 12 hours in a water bath. This process is called exfoliation [
Graphene oxide was reduced by using ascorbic acid as a reducing agent. 400 mg of GO powder was dispersed in 400 mL of distilled water (0.1 mg·mL−1). Next, 4 g of ascorbic acid (AA) was added to this solution and stirred with a magnetic stirrer for 30 min at 60°C; the reduced product was centrifuged at 4000 r/s for 40 min to remove the supernatant. Then, excess 30 wt.% H2O2 was added to the black paste to oxidize the remaining ascorbic acid by stirring for 30 min at 60°C. After stirring, the resultant black product was collected by centrifugation at 4000 r/s, washed with ethanol and distilled water 3 times, respectively, and dried at 120°C for 24 hours. The preparation process of reduced graphene oxide (rGO) consists of three steps: oxidation or intercalation, exfoliation, and reduction. Oxidation or intercalation: graphite oxide (GTO) was prepared from graphite powder by reacting with a strong oxidizing agent called potassium permanganate mixed with H2SO4/H3PO4 (9 : 1) (v/v) at 50°C for 12 hours in a water bath. H2SO4 is the most common intercalating agent. Exfoliation: in this step, the oxidized form of graphite was dispersed into distilled water to form single-layer graphene oxide (GO), and then it was heated with a magnetic stirrer for 12 hours at 60°C in a water bath. Then, the black paste was collected by filtration followed by centrifugation and dried at 60°C for 24 hours. Reduction: it is the last stage in synthesizing reduced graphene oxide (rGO). In this stage, the GO obtained from stage (ii) was dispersed and ascorbic acid was added as the reducing agent, followed by heating it at 60°C for 30 min. The reduced product was collected using filtration followed by centrifugation. Lastly, the black product was washed with ethanol and distilled water three times, respectively, followed by centrifugation; then, the product was collected and dried in an oven at 120°C for 24 hours.
Visual characterization means directly observing the changes in GO before and after reduction. The color change in the sample was captured by using a video camera. A UV-visible spectrometer (PerkinElmer, USA) was used to evaluate the photocatalytic activities of the sample. The GO and rGO solution samples were scanned at a wavelength ranging from 200 nm to 800 nm. FTIR (PerkinElmer, USA) was employed to analyze the presence/absence of functional groups on GO and rGO sheets. GO and rGO pellets were prepared using KBr, and the samples were scanned in the range from 400 cm−1 to 4000 cm−1 to obtain the FTIR spectra. The X-ray diffraction (XRD Philips X-ray diffractometer) patterns of GO powders were recorded with a scanning rate of 1° per minute in a 2
The color changes were simply observed when chemicals were added. These color changes were captured by using a video camera. Scheme
Steps of graphene oxide and reduced graphene oxide synthesis.
The paste can cling on to the glass rod. In mixing process, potassium permanganate was gradually added with continuous stirring to control the reactivity. If the oxidizing agent is added once, it forms dark brown ash in gaseous form. At the end of 12 hours of boiling at 50°C with a magnetic stirrer, the greenish dark product was converted into paste (Scheme
The suspension with metal ions and acids is washed with 5% HCl solution to remove the metal ions [
Visual characterization of (a) GTO solution and GO solution, film, and powder and (b) rGO solution, film, and powder. Solutions in (a) and (b) were formed at the 1 : 1 wt./v ratio of the sample with distilled water as the solvent, and their images were taken using a video camera.
During the oxidation of graphite, oxygen attaches to the graphene layers, thereby increasing the polarity of the layers which in turn increases their solubility in water. This results in a change in the color of the solution from yellow to brown; depending on the concentration of GO, brown color can be of different intensities. The GO that is formed has an absorption maximum at 225 nm for a well-oxidized material [
As the Beer–Lambert law states that absorbance is directly proportional to concentration, a sample with greater concentration has higher absorption than a sample with smaller concentration. Samples 1, 2, 3, and 4 indicated in Table
Synthesis of GO at different mixture weight ratios of graphite and potassium permanganate.
Sample | Graphite (g) | KMnO4 (g) | H2SO4/H3PO4 ratio (v/v) | H2O2 (mL) | Temperature (°C) | Time (hours) |
---|---|---|---|---|---|---|
1 | 0.5 | 3.5 | 9 : 1 | 10 | 50 | 12 |
2 | 0.5 | 4.0 | 9 : 1 | 10 | 50 | 12 |
3 | 0.5 | 4.5 | 9 : 1 | 10 | 50 | 12 |
4 | 0.5 | 5.0 | 9 : 1 | 10 | 50 | 12 |
UV-Vis spectra at different concentrations.
The difference in oxidation behavior of graphite is widest at lowest temperature, tending to disappear as the temperature increases. The oxidation of graphite at constant concentration and time is highly temperature dependant. As temperature increased, the rate of oxidation increased rapidly. At 40°C, it shows absorbance at 230 nm; at 50°C, it shows absorbance at 224 nm; and at 60°C, it shows absorbance at 227 nm, which is indicated in Table
Synthesis of GO at different reaction temperatures.
Sample | Graphite (g) | KMnO4 (g) | H2SO4/H3PO4 ratio (v/v) | H2O2 (mL) | Temperature (°C) | Time (hours) |
---|---|---|---|---|---|---|
6 | 0.5 | 3.5 | 9 : 1 | 10 | 40 | 12 |
7 | 0.5 | 3.5 | 9 : 1 | 10 | 50 | 12 |
8 | 0.5 | 3.5 | 9 : 1 | 10 | 60 | 12 |
UV-Vis spectra at different temperatures.
Reaction time is an essential factor for oxidation and reduction. It is found that the carbon content increased with increasing annealing time. Samples 8, 9, and 10, as shown in Table
Synthesis of GO at different reaction time periods.
Sample | Graphite (g) | KMnO4 (g) | H2SO4/H3PO4 ratio (v/v) | H2O2 (mL) | Temperature (°C) | Time (hours) |
---|---|---|---|---|---|---|
6 | 0.5 | 3.5 | 9 : 1 | 10 | 50 | 10 |
7 | 0.5 | 3.5 | 9 : 1 | 10 | 50 | 12 |
8 | 0.5 | 3.5 | 9 : 1 | 10 | 50 | 14 |
UV-Vis spectra at different time periods.
Graphene oxide (GO) that was obtained from the oxidation of 0.5 g graphite with 4.5 g of potassium permanganate and (90 : 10) mL v/v ratio of sulfuric and phosphoric acids was reduced by 4.0 g ascorbic acid to get reduced graphene oxide (Scheme
Schematic representation of reduced graphene oxide.
UV-Vis spectra of GO and rGO.
After reduction by using AA, the peak of reduced graphene (the red curve) was observed at 257 nm. The absorption of reduced graphene (red curve) shifts from 226 to 256 nm, suggesting that the electronic conjugation within graphene sheets is restored after the reduction [
In order to examine the efficiency of introduction of oxygen-containing functional groups into carbon lattice, FTIR spectroscopy was used. At each spectrum, several typical modes corresponding to oxygen-containing functional groups were detected. Here, stretching vibration modes of C–O bonds arise at 1,078 cm−1. The peaks between 1,190 cm−1 and 1,382 cm−1 are related to C–OH stretching vibration [
When reducing graphene oxide to graphene, the functional groups disappeared. This is clearly reflected in the FTIR spectrum (Figure
FTIR spectra of GO and rGO.
X-ray diffraction patterns for graphite powder, GO powder, and rGO powder were recorded. As shown in Figure
X-ray diffraction patterns of (a) graphite powder, (b) GO powder, and (c) rGO powder.
Synthesis of graphene oxide was achieved by placing graphite in concentrated acid in the presence of an oxidizing agent. The Tour method is a less hazardous and more efficient method for graphite oxidation because it is cost-effective, nontoxic, and environmental friendly. This is the most commonly used methods for the oxidation of graphite. Graphene oxide was reduced by chemical reduction process using AA (ascorbic acid) that works both as a reducing agent and a protecting agent, which makes the process economical, nontoxic, and environmental friendly.
The oxidation level of graphite powder was observed using different parameters such as reaction time, temperature, and quantity of KMnO4. Graphene oxide (GO) and reduced graphene oxide (rGO) both were characterized by different characteristic techniques. These techniques include visual observation, UV-Vis characterization, FTIR characterization, and XRD (X-ray diffraction) techniques. UV-Vis spectroscopy showed formation of GO and rGO with maximum absorption peaks at 226 nm and 257 nm, respectively. FTIR spectroscopy spectra showed the introduction of different oxygen-containing functional groups in GO during oxidation and removal of these functional groups after reduction. XRD also showed that the diffraction peak of GO (2
All data generated or analyzed during this study are included within the article.
The authors declare that they have no conflicts of interests.
The authors express their sincere gratitude to their advisor, Dr. Delele Worku, former Dean of Science College of Bahir Dar University, for his visionary guidance, insightful advises, and genuine supports.